Reactivity and selectivity of bowl-shaped polycyclic aromatic hydrocarbons: Relationship to C₆₀

The Diels-Alder reactivity of different bowl-shaped polycyclic aromatic hydrocarbons (namely, corannulene, cyclopentacorannulene, diindenochrysene, hemifullerene, and circumtrindene) has been explored computationally within the DFT framework. To this end, both the increase in reactivity with the size of the buckybowl and complete [6,6]-regioselectivity in the process have been analyzed in detail by using the activation strain model of reactivity in combination with the energy decomposition analysis method. These results have been compared with the parent C₆₀ fullerene, which also produces the corresponding [6,6]-cycloadduct exclusively. The behavior of the buckybowls considered herein resembles, in general, that of C₆₀. Whereas the interaction energy between the deformed reactants along the reaction coordinate mainly controls the regioselectivity of the process, it is the interplay between the activation strain energy and the transition-state interaction that governs the reactivity of the system. Bigger bowls, better reactivity: Starting from corannulene, there is a smooth convergence to the C₆₀ energy barrier and reaction energy for the Diels-Alder reaction with cyclopentadiene when the size of the buckybowl is increased (see figure). Through density functional calculations, the origins of both this trend of reactivity and the observed exclusive [6,6]-regioselectivity are analyzed in detail.